feat(repo-cache): sync pre-fetched repositories without listing the organization #28

Problem

Following #27, repo_cache.sync_org() correctly reports empty repositories separately from synchronization failures. However, it always lists the organization through gh and accepts only repository-name filters.

Consumers such as generate-repo-overview already fetch the complete repository set through PyGithub. They need to synchronize those same repositories, but using sync_org() causes a second organization listing and loses the already-fetched repository records.

Proposed change

Add a high-level synchronization API that accepts repository records supplied by the caller, for example:

sync_repositories(
    *,
    org: str,
    repositories: Sequence[Repository],
    cache_dir: Path,
    workers: int = DEFAULT_SYNC_WORKERS,
    progress: Callable[[str], None] | None = None,
) -> SyncReport

sync_org() should retain its current behavior and delegate to this API after listing and filtering repositories.

Acceptance criteria

• Callers can synchronize a pre-fetched sequence of repo_cache.Repository records without another organization-list API request.
• The returned SyncReport preserves the post-feat(repo-cache): handle empty repositories #27 empty-repository and failure classification.
• Checkout paths and outcome ordering remain deterministic and compatible with sync_org().
• sync_org() behavior remains unchanged and shares the implementation.
• The library documentation includes the pre-fetched-repository use case.

This would let downstream collectors fetch GitHub metadata once, synchronize checkouts once, and pass the resulting paths into their own processing workers.